Chiral metallocenes are useful for the synthesis of polyolefins. The racemic form of these metallocenes provides stereoregular polymers, e.g., poly(alpha-olefins). In addition, racemic metallocenes are considerably more active as catalysts than the meso metallocenes, which tend to produce only low molecular weight atactic polymers. Metallocenes and their use as catalysts in forming various olefin polymers are described, for example, in U. S. Pat. Nos. 4,794,096; 5,017,714; 5,036,034; 5,145,819; 5,296,434; 5,324,800; 5,329,033; 5,455,365; and 5,455,366, the full disclosures of which are incorporated herein by reference.
Processes for the synthesis of such metallocenes tend to form mixtures of the meso and racemic forms. One way of obtaining the racemic metallocene is to carry out a physical separation of the racemic and meso forms in such mixtures. For example use can be made of crystallization or extraction procedures such as described in EP 344 887, or Huhn, et al., Chem. Lett., 1997, 12, 1201-1202; or by chromatographic procedures such as described by Wild, et al., J. Organomet. Chem., 1982, 232, 233-247 or by Grossman, et al., J. Am. Chem. Soc., 1991, 113, 2321-2322. However such procedures are not well suited for use on an industrial scale because of their expense and low yield.
Other methods for separating the racemic and meso metallocene forms have been proposed. Thus EP 819 695 (published Jan. 21, 1998) describes modifying the racemic/meso ratio of a racemic/meso mixture of a stereorigid, bridged metallocene by subjecting the mixture to selective decomposition of the undesired isomer in the presence of compounds having either acidic hydrogen atoms reactive halogen atoms, such as water, methanol, ethanol, acetic acid, hydrochloric acid, chlorotrimethylsilane, or p-toluenesulfonic acid. For example, use of water with a 1:1 mixture of rac/meso 1,2-ethylene bis(4,7-dimethylindenyl)zirconium dichloride gave 100% of the meso form, and thus destroyed the racemic form.
Another approach involves photochemical isomerization procedures. Thus DE 19,525,178, DE 19,525,184, WO 97/03080, and Schmidt, et al., Organometallics, 1997, 16(8), 1724-1728 describe procedures involving irradiation of meso metallocenes or rac/meso metallocene mixtures into racemic forms in the presence of a chiral reagent such as the R(+) enantiomer of dilithium binaphtholate.
It would be most advantageous if a way could be found for effectively treating meso metallocenes or mixtures of meso and racemic metallocenes so as to produce a product enriched in the desirable racemic form without use of extraction, crystallization, or chromatographic separations, without destruction of one of the isomeric forms, and without need for use of photolysis apparatus and chiral reagents.
U.S. Pat. No. 5,780,660 (issued Jul. 14, 1998) describes an isomerization process in which a slurry of (i) a mixture of meso and racemic forms of a bridged zirconocene in (ii) a liquid ether-containing isomerization medium, is maintaining at one or more temperatures, preferably 30 to 70.degree. C., for a period of time such that a racemate enriched zirconocene is formed.